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Authors: John Sabau, (Insoil Canada Ltd, Canada) Issouf Fofana, (ISOLIME, UQAC, Canada) S Gal, ( Transelectrica, Romania ) M. Oltean ( SC Smart SA, Romania). Inoxidable Oil-Paper Insulation in Free Breathing Power Transformers. Introduction. Power transformers

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Authors: John Sabau, (Insoil Canada Ltd, Canada)

Issouf Fofana, (ISOLIME, UQAC, Canada)

S Gal, (Transelectrica, Romania)

M. Oltean (SC Smart SA, Romania)

Inoxidable Oil-Paper Insulation

in Free Breathing Power Transformers


Power transformers

the most valuable asset in a substation or plant,

Indispensable components of high-voltage equipment for power generation plants, transmission systems and large industrial plants.

Unexpected failures

 major disturbances to operating systems,

unscheduled outages and power delivery problems.

Such failures can be the insulation degradation/aging, result of poor maintenance, poor operation, poor protection, undetected faults, or even severe lightning or short circuits

Outages affect revenue, incur penalties and can cost a company its reputation and its customers.

in extreme cases, they can leave a company facing financial ruin

the problem
The Service Reliability of aging transformers decreases while the number of failing transformers is growing

Aging Power Transformers pose an increased risk of failures and blackouts

“Power Transformers built in the ’60s and ‘70s pose a serious strategic issue worldwide. Catastrophic failures are escalating both in number and cost including environmental impacts”

IEEE Electrical Insulation Magazine 2001

The problem
the consequence
The consequence

*Cost estimates of an unplanned replacement of a typical generator step-up transformer

  • Transformer failures can cost up to $15 million, in addition to an operator’s reputation.
  • Incentive for electric companies to ensure reliability and availability throughout the life cycle of these key assets.

*Doble Life of a Transformer Seminar - Clearwater, FL, United States

oil decaying 1
Oil Decaying (1)






  • Some of the hydrocarbon bonds may break and form free radicals: H*, CH3*, CH2* and CH*,… which then recombine to form:
  • soluble/insoluble decay products
  • fault gases observed in oil.
the dissolved oxygen
The dissolved Oxygen

Oil in contact with air at atmospheric pressure dissolves 10% air by volume (Henry’s law).

The dissolved oxygen content of an oil sample, taken from the tank of a free-breathing transformer, shows values, between 5,000 and 40,000 ppm

The omnipresence of oxygen(with the catalytic effect of copper), heat and water will promote insulating liquid oxidation even under normal conditions

According to Fabre and Pichon [1] oxygen increases the rate of degradation of paper in oil containing 0.3-5% moisture by a factor of 2.5 and water increases the rate in simple proportion to its concentration.

Reducing the oxygen from saturation level in the oil (30,000 ppm) to less than 300 ppm reduces the ageing by a factor of 16!

[1] J. Fabre and A. Pichon, “Deteriorating processes and products of paper in oil. Application to transformers”, CIGRE, Paper 137, pp. 1-18, 1960.

limitations of current preventive procedures
Limitations of current preventive procedures

Aggressiveness of oxygennegatively affects the chemical stability of oil by generating decay products (detrimental to the solid insulation)

researchers have developed various technical solutions.

Nitrogen sealed type Conservator (Fig 1)

Diaphragm sealed type Conservator (Fig 2)

The Use of Oxidation Inhibitors


Rubber bladder

Oil level indicator


Fig 1

Transformer tank

Dehydrating breather

Fig 2

dissolved oxygen and moisture removal system
Dissolved Oxygen and Moisture Removal System










To shorten the experimental time frame, a relatively small unit was suitable.

Two 5 MVA distribution transformer were utilized

dissolved oxygen and moisture removal system1
Dissolved Oxygen and Moisture Removal System
  • this innovative nitrogen blanketing system gradually decreases the dissolved oxygen from insulating oil over the experimental period, while maintaining the “freely breathing” character of the transformer.
transformer 1
Transformer #1

Dissolved gas Analysis and relative decay products and Turbidity results after 8 years blanketing.

The level of dissolved carbon dioxide and combustible fault gases also decreased. The absorbance curves along with the turbidity of the 5 MVA blanketed unit show little change.

transformer 2
Transformer #2

Dissolved gas Analysis and relative decay products and Turbidity results after 8 years blanketing.

These analyses confirm the theoretical premises that motivated the experiment.

additional testings
the Total Acid Number (TAN) shows slight change while the interfacial tension (IFT) is still not a reason for concern.

However, an increase the dissipation factor is observed. This might be related to the ability of free radicals to capture a free electron and become a charge carrier

Additional testings
economic and technical considerations
Dissolved Oxygen and Moisture Removal System (DORS)

Enhances the service reliability

Eliminates oxidation decay

Reduces the cost of maintenance

Proven technology

Low cost of deployment

Lower maintenance cost

Enhanced service reliability (of current transformers)

Excellent ROI

Economic and Technical considerations
economic and technical considerations1
Contrary to breathing units:

Since the nitrogen supplied by the membrane generator is moisture free and its flow is in one direction only, frequent changing of depleted silicagel in the air desiccators filters is no longer necessary.

With the removal of dissolved oxygen in the oil, secondary chemical reactions with the gases generated under the effect of electrical stress become impossiblewhich might therefore allows a better reproducibility and interpretation of dissolved gas analysis (DGA).

The expenses imposed by the reclamation and reconditioning of the oil may be reduced once the very cause of oxidation of the decay process affecting the composite solid-liquid insulation is reduced.

The use of antioxidant inhibitors may be unnecessary, including the laboratory analysis to monitor their declining concentration and the costs involved in periodic replenishment.

Economic and Technical considerations
economic considerations
Contrary to sealed or rubber bladder units:

Fault gases can freely escape from the transformer.

The system is reparable in situ.

Old transformers may be easily retrofitted.

 The life expectancy of aging transformers is extended. Among all, the low initial cost and almost maintenance free operation of membrane nitrogen generators makes this prevention technique economically affordable.

Economic considerations

Unlike the current use of oxidation inhibitors which are able only to retard the oil-paper insulation deterioration process; this economically affordable procedure successfully prevents the premature aging of these expensive machines

Rather than reconditioning or reclaiming service-aged oil in order to restore its initial properties, the Dissolved Oxygen and Moisture Removal System (DORMS) open the possibility of maintaining the initial physical and chemical properties of a liquid dielectric throughout the lifetime of a power transformer

By using this system, the oxidation decay of oil-paper insulation will be hindered and the life expectancy of these expensive machines considered capital investments in every country infrastructure, extended, while the mineral insulating oil, a non-renewable resource, could be given a practically endless life expectancy.


Thank you…

Questions & Answers?